1. Field of the Invention
The present invention relates to methods for manufacturing Nb3Sn superconducting wires by the powder metallurgy process. In particular, the invention relates to a method for manufacturing a powder-metallurgy processed Nb3Sn superconducting wire useful as the material of superconducting magnets used for high magnetic field generation, and to a precursor to the powder-metallurgy processed Nb3Sn superconducting wire.
2. Description of the Related Art
Among the fields using superconducting wires in practice are superconducting magnets used in high-resolution nuclear magnetic resonance (NMR) spectrometers. The higher magnetic field a superconducting magnet generates, the higher resolution is achieved. Accordingly, superconducting magnets capable of generating higher magnetic field are desired more and more.
For example, Nb3Sn wire is in practical use as the superconducting wire used for the superconducting magnet for high magnetic field generation. The Nb3Sn superconducting wire is generally manufactured by the bronze process. In the bronze process, Nb-based cores are buried in a Cu—Sn-based alloy (bronze) matrix and drawn into filaments. The filaments are bundled together to use as the material of the superconducting wire. The bundle of the filaments is buried in copper for stabilization (stabilizing copper), and drawn into a wire.
The resulting wire is subjected to heat-treatment (diffusion heat treatment) at 600 to 800° C., thereby forming a Nb3Sn phase at the interfaces between the Nb-based filaments and the matrix. Unfortunately, this process limits the content of Sn turning into a solid solution in bronze (to 15.8 mass % or less), and accordingly the resulting Nb3Sn phase has a small thickness. Also, the crystallinity of the Nb3Sn is degraded and the properties in high magnetic fields are inferior.
In addition to the bronze process, a tube process and an internal diffusion process are also known as methods for manufacturing Nb3Sn superconducting wire. The tube process has disclosed in, for example, Japanese Unexamined Patent Application Publication No. 52-16997. In this process, a Nb tube containing a Sn core is inserted in a Cu pipe and subjected to diameter reduction, followed by heat treatment. Thus, the Nb and the Sn diffuse and react with each other to produce Nb3Sn. The internal diffusion process has been disclosed in, for example, Japanese Unexamined Patent Application Publication No. 49-114389. In this process, a Sn core is buried in the center of a Cu base material. A plurality of Nb wires are placed in the Cu base material around the Sn core. After diameter reduction, the Sn is diffused by heat treatment to react with the Nb, thereby producing Nb3Sn. These processes have no limit of the Sn content, unlike the bronze process, which limits the Sn content due to the solid solubility limit. Accordingly, the Sn content can be set as high as possible to enhance the superconducting properties of the resulting wire.
The Nb3Sn superconducting wire may be manufactured by another process, a powder metallurgy process. For example, in Japanese Unexamined Patent Application Publication No. 5-290655, a Nb or Nb alloy sheath is filled with a mixture of Cu powder and Sn powder as a core (powder core). After diameter reduction by, for example, extrusion or wiredrawing, the material is subjected to heat treatment (diffusion heat treatment). In this process, the surfaces of the Sn powder particles may be coated with Cu plating in order to ensure the flowability of the powder mixture. In order to enhance the characteristics of the superconducting wire, for example, Japanese Unexamined Patent Application Publication No. 5-28860 has disclosed a technique in which Ti, Zr, Hf, Al, Ta, or the like is added to the powder mixture of Cu and Sn. These processes can produce a thicker and more high-quality Nb3Sn phase than the bronze process, and are accordingly expected to produce superconducting wires with superior high magnetic field properties and allow a high Sn content in the powder mixture.
FIG. 1 is a schematic sectional view of a state in the course of manufacture of a powder-metallurgy processed Nb3Sn superconducting wire, wherein reference numeral 1 represents the Nb or Nb alloy sheath; reference numeral 2 represents a powder core formed of a raw material powder packed in the sheath 1; and reference numeral 3 represents a stabilizing copper (Cu matrix). In the powder metallurgy process, the sheath 1 is filled with a raw material powder containing at least Sn to form the powder core 2, and the sheath 1 is placed in the stabilizing copper 3, followed by diameter reduction performed by, for example, extrusion or wiredrawing. The resulting wire is wound around a magnet or the like and subsequently heat-treated, so that a Nb3Sn superconducting phase is formed at the internal surface of the sheath 1.
The raw material powder must contain Sn. However, if Sn is contained in a form of powder, the Sn powder has a low melting point and, accordingly, may melt out by heat of extrusion or wiredrawing. Also, the raw material powder containing Sn powder is disadvantageously difficult to anneal during extrusion or wiredrawing. Furthermore, the Cu powder and the Sn powder have different specific gravities and grain sizes. It is therefore difficult to uniformly mix these powders. Consequently, Cu—Sn alloys or compounds are nonuniformly produced in the material during heat treatment and cause the material to break.
In view of these disadvantages, another technique has been proposed in which Sn is alloyed in advance. For example, Japanese Unexamined Patent Application Publication No. 5-342932 has proposed a process in which breakage or other problems resulting from the segregation of the Cu—Sn alloy or compound can be prevented by filling a Nb or Nb-based alloy sheath with a previously prepared compound (or alloy) powder of Cu and Sn to form the core (powder core).
While it is considered that the heat treatment for forming the superconducting phase is preferably performed at a high temperature of about 900 to 1000° C., it is known that the presence of Cu in the raw material powder can reduce the heat treatment temperature to about 650 to 750° C. The Cu contained in the raw material powder is intended for this effect. Incidentally, although the core schematically shown in FIG. 1 is single, a plurality of cores are generally placed in the Cu matrix in practice.
For use of a Cu—Sn alloy or intermetallic compound powder as the raw material powder, a Cu powder and a Sn powder are weighed out and mixed, and the mixture is heat-treated and then pulverized. However, the powder thus prepared (hereinafter may be referred to as Cu—Sn compound powder) is so hard and brittle as to make it difficult to fill the sheath uniformly, and the percentage of the packed powder becomes low.
In general, the raw material powder is packed into the sheath by uniaxial press. On the other hand, it is considered that the percentage of the packed powder can be increased by isotropic compaction, such as cold isostatic press (CIP), and that the isotropic compaction is effective in producing a uniform wire. However, even if the CIP is applied to the Cu—Sn compound powder, the resulting compact is brittle and easy to crack or break. It is thus difficult to fill the sheath. From the viewpoint of increasing the strength of the compact, hot isostatic press (HIP) may be useful. This technique, however, causes the Cu—Sn compound powder particles to be bound to each other. Consequently, the cutting workability of the resulting compact is enhanced, but its plastic workability is degraded. Thus, the extrusion and wiredrawing of the compact becomes difficult.